1. Field of the Invention
The present invention relates to cellular wireless networks, and in particular to orthogonal frequency division multiplexing access (OFDMA) equipment and methods to reduce interference caused by competing adjacent base-station transmitters. Currently, such OFDMA equipment includes IEEE-802.16 WiMax, IEEE-802.20 OFDMA, and the next generation of 3G systems.
2. Description of the Prior Art
The nature of radio communication is there will always be some sort of interference. Serious interference will be disruptive and can make communication impossible. Its sources can either be other communication or merely the unintentional spurious radiations of nearby electronic devices. Less serious interference can cause data errors and inject noise into video and audio channels.
Channel and bandpass filters can be very effective at both the source of the interference and the receiver to control the adverse effects. But if the interference is on the same channel frequency, and is relatively close, then it becomes impossible to avoid it.
Cellular type communications systems place many similar base-stations with their radio transmitters side-by-side in the cellular matrix in a region. A single base-station can be completely surrounded at the fringes of its service area by several other base-stations in the same service. Such allows one base-station to handover its support to another as mobiles roam through the agglomeration of service cells.
At some point between every two or three such base-station radio transmitters, a mobile roaming in the region will receive all equally. It becomes problematic at that point which one to listen to, and how to ignore or filter-out the base-stations of no interest.
In real commercial applications, there are too many users and too few channels not to make every frequency, time, or code division multiple access slot available in every cell. Schemes that dispose of slots to end interference must do so very judiciously, and then only as long as circumstances justify the waste. Otherwise, “dial-tone” and other indications of service availability will be scarce and the client users will be annoyed or alienated.
Edward Teague describes “Interference Control Via Selective Blanking/Attenuation of Interfering Transmission” in United States Patent Application US 2005/025455 A1, published Nov. 17, 2005. Neighboring sectors use a target user's frequency hopping sequence to form a blanking pattern. Subbands that overlap on a particular roaming-user are either blanked or power is reduced to quell the interference that would otherwise occur. The receivers then see either punctured or lower energy symbols for these subbands. The blanking and power reductions are transparent to the target and neighboring users, and so do not need to do anything special with the involved subbands.
Such does not use a backbone or central controller 108 to coordinate and track all the blanked or de-powered subbands in all the base-stations and users in a region. Also, it suggests blanking co-ordination between base stations at a subscriber station level. The unavoidable latencies in recognizing the data traffic patterns of subscriber stations, scheduling of bandwidth for subscriber stations by interfering base stations and the latencies involved in communicating between all the interfering base stations, makes the scheme impractical or less efficient. Communications latencies prohibit dealing with individual terminal devices on a case-by-case basis. Service is typically demanded more quickly than the control decisions can be made.
Code division multiple access (CDMA) modulation is the principle reason cellular transmissions don't collide with one another when two different transmitters broadcast at the same time and on the same frequency. But the new OFDMA technology making a start this year and next, uses modulation techniques that require relatively clear channels.
So-called WiMax actually involves two different IEEE Standards. A key difference between IEEE-802.16-2004 and IEEE- 802.16e profiles is their multiplexing techniques. IEEE-802.16-2004 uses OFDM, and IEEE-802.16e uses OFDMA. OFDM works by splitting the radio signal into multiple smaller sub-signals that are then transmitted simultaneously at different frequencies to the receiver. It can reduce the amount of crosstalk in signal transmissions.
For broadband wireless access, most of the activity takes place in the Media Access Control (MAC) layer and the Physical (PHY) layer. Broadband wireless air (PHY) interfaces include single carrier, Orthogonal Frequency Division Multiplexing (OFDM), and Orthogonal Frequency Division Multiple Access (OFDMA). Others are Wideband Code Division Multiple Access (WCDMA), Universal Mobile Telecommunications System (UMTS), cellular 3G, and the many variations of OFDMA being considered for 4G and future indoor wireless networks.
Wi-Fi generally refers to the 802.11a/b/g/n family of indoor wireless networks. Many vendors built proprietary MAC and PHY systems that extended these capabilities to outdoor networks. Some of these systems used a single carrier. Several leveraged OFDM capabilities. Others chose WCDMA or UMTS approaches. But the idea was to create effective outdoor networks.
The IEEE-802.16 defines a metropolitan area network (MAN) standard for broadband wireless or WiMAX, e.g., fixed broadband wireless (802.16-2004), and mobile broadband wireless (802.16e). Both support multiple PHY modes, neither of which include WCDMA or UMTS.
802.16-2004 (fixed)802.16e (mobile)Single CarrierSingle CarrierOFDM 256 FFTOFDM 256 FFTOFDMA 2048 FFTOFDM 2048 FFTsOFDMA 1024 FFTSOFDMA 512 FFTsOFDMA 128 FFT
The WiMAX Forum chose the OFDM 256 FFT mode for the fixed WiMAX product. Product profiles for mobile WiMAX have yet to be chosen as the standard is not yet ratified. Some version of OFDMA is expected to be incorporated. The Qualcomm Flarion uses a variation called Flash OFDM.
WiMax transmissions use a combination of frequency (FDMA) and time division (TDMA) techniques to increase the number of mobiles that can be supported while roaming in each service area. The scarcity and licensing expense of the radio spectrum used for WiMax service means neighboring cells will be using the same frequencies.
So what is needed is a cellular wireless network that can make optimal use of scarce and expensive broadband wireless air resources with ever-changing populations of remote, subscriber terminal, and roaming mobile user devices.